The present invention relates to a melt-extrudable thermoplastic composition which contains an additive system, and to shaped articles made therefrom.
Thermoplastic compositions are described in U.S. Pat. No. 4,923,914 to Ronald S. Nohr and J. Gavin MacDonald, incorporated herein by reference, which include at least one thermoplastic polymer and at least one defined additive. The additive particularly is a siloxane-containing compound. The compositions are especially useful for the formation of nonwoven webs by such melt-extrusion processes as meltblowing, coforming, and spunbonding. Upon being melt-extruded, such compositions result in a fiber having a differential, increasing concentration of the additive from the center to the surface thereof, such that the concentration of additive toward the surface of the fiber is greater than the average concentration of additive in the more central region of the fiber and imparts to the surface of the fiber at least one desired characteristic which otherwise would not be present. The additive is miscible with the polymer at melt extrusion temperatures, under which conditions the additive and the polymer form a metastable solution. As the temperature of the newly formed fiber drops below melt extrusion temperatures, the additive becomes significantly less compatible with the polymer. Concurrent with this marked change in compatibility, the polymer begins to solidify. Both factors contribute to the rapid migration or segregation of the additive toward the surface which takes place in a controllable manner.
It subsequently was discovered that the use of an additive system comprising certain polysiloxane polyether additive s of U.S. Pat. No. 4,923,914 and a modified fumed silica having a hydrophobic surface unexpectedly gave a nonwoven web which was not wettable, but exhibited improved tensile strength characteristics as compared with a nonwoven web prepared from a thermoplastic composition lacking the hydrophobic fumed silica. This discovery is part of the disclosure of cross-referenced application Ser. No. 07/783,438 filed Oct, 25, 1991, U.S. Pat. No. 5,344,862. However, a further improvement of tensile strength characteristics, exceeding the improvements taught by application Ser. No. 07/783,438, filed Oct 25,1991, U.S. Pat. No. 5,344,862 now has been discovered.
Silica and other materials have been incorporated into thermoplastic polymers, including polypropylene. For example, the inclusion of an organic peroxide and a nucleating agent in polypropylene is described in Japanese Patent Publication No. 60-217207. The nucleating agent can be an organic salt, an organic compound, or an inorganic material such as silica, alum, titanium dioxide, carbon black, and various clay minerals.
References which describe the inclusion in polypropylene or other thermoplastic polymer of an organic material include U.S. Pat. Nos. 3,207,735 to Wijga (benzoic acid, substituted benzoic acids, hexahydro analogs thereof, and related compounds), 3,207,737 to Wales (aluminum salts of benzoic acid and related compounds), 3,207,739 to Wales (Group I and II metal salts of certain mono- and polycarboxylic acids), 3,299,029 to Binsbergen et at. (aluminum salts of benzoic acid and related compounds), 4,611, 024 to Wolfe (an acetal of an alditol and a hydrotalcite), and 4,808,650 to Titus et al. (fluorinated dibenzylidene sorbitol additives); and Japanese Patent Publication No. 51-22740 (benzylidene sorbitol).
Finally, studies relating to the heterogeneous nucleation of polymers have been reported. Examples of such studies include Chatterjee and Price, "Heterogeneous Nucleation of Crystallization of High Polymers from the Melt. I. Substrate-Induced Morphologies", J. Polym. Sci.,13,2369 (1975); Collington, "The Nucleation of Crystalline Olefins", Polypropylene: The Way Ahead, a conference of the Plastics and Rubber Institute, Madrid, Spain, November 1989; and Garg and Stein, "Crystallization and Morphology of Nucleated Polymers", Antec '88, 1021.
Notwithstanding the foregoing, it should be noted that neither the siloxane copolymers (whether polysiloxane polyethers or alkyl-substituted polysiloxanes) nor a modified fumed silica, when used alone, gave any improvement in tensile strength characteristics. For reasons not yet fully understood, there appears to be a synergy which results from the use of the modified fumed silica with either a polysiloxane polyether as taught by application Ser. No. 07/783,438 filed Oct 25,1991, U.S. Pat. No. 5,344,862, or an alkyl-substituted polysiloxane as taught herein.
Without wishing to be bound by theory, it is believed that the alkyl-substituted polysiloxane serves three functions: (1) it acts as a dispersing agent for the modified fumed silica, thereby reducing or preventing flocculation of the silica into larger particles, especially after destructuring; (2) it helps reduce the surface free energy of the modified fumed silica which results in coated silica surfaces which are more readily "wet" by molten polyolefin; and (3) it acts as a processing aid for the entire system during the melt-extrusion process.